Acoustic resonators for precise selection and control of radio frequencies are well known in high sensitivity sensor circuitry, such as accelerometers. The two most common types of acoustic vibrators are the bulk acoustic wave devices (BAW) and surface acoustic wave devices (SAW).
A perspective view and a side view of a typical BAW are shown in FIGS. 1 and 2 respectively. The BAW includes a piezoelectric plate as a substrate 100, a top electrode 102 on one surface of the substrate and a bottom electrode 104 on the opposite surface. When the electrodes 102, 104 are energized, usually by RF signals, an electric field is created. The mechanical forces inherent to the electric field stress the piezoelectric substrate producing a pattern of strain known as a mechanical mode shape.
The alternating RF signal creates vibration in the mechanical mode shape, resulting in acoustic waves. The acoustic waves travel through the thickness of the piezoelectric plate, reflecting at the surfaces and setting up standing waves throughout the interior of the plate. The acoustic waves are confined essentially to the active region 106 between the two electrodes 02, 104, and drop off sharply outside of the region occupied by the electric field created by the energized electrodes 102, 104. The frequency of a BAW is expressed approximately as ##EQU1## where n is the overtone number, an odd integer (1, 3, 5 . . .)
t is the plate thickness PA1 .rho. is the material density PA1 c is the piezoelectrically stiffened elastic constant
In general, n, t and .rho. will be invariant during operation. However, c may vary as the result of a number of factors, thus altering the frequency of the BAW.
A typical surface acoustic wave device (SAW), is shown in FIG. 3. Energized electrodes create a mechanical mode shape and resulting acoustic waves, similar to a BAW. However, the electrodes 202, 204 of a SAW are arranged on only one side of piezoelectric substrate 200, and the acoustic waves are confined to the surface of the substrate 200 having the electrodes. As with the BAW, the RF signal electrodes are arranged in sets. However, the acoustic waves travel only in the area of the surface of the piezoelectric substrate bounded by RF signal electrodes 202, 204 and grating reflectors 206. The edges of the substrate outside this area are essentially at rest.
The first set of electrodes 202 is fed by a voltage source 208 and the second set of electrodes 204 is grounded. The electrodes are configured to have bus bars such as 202b, 204b and fingers such as 202a, 204a. The electrode sets 202, 204 are interdigitated as shown so that acoustic waves move from the fingers of one electrode set to adjacent fingers of the second electrode set. The grating reflectors 206 are used to control the extent of acoustic wave propagation over the surface of the piezoelectric substrate. Additional sets of electrodes 210, 212 and grating reflectors 214 can be arranged on the surface of the piezoelectric substrate according to the demands of the system containing the SAW.
The frequency of a SAW is roughly expressed as: ##EQU2## where d is the inter-digitated finger spacing for both the interdigitated transducers (IDT) and the grating reflectors, c is similar to the piezoelectrically stiffened elastic constant used in the formula for the BAW, and .rho. is similar to the constant used in the formula for the BAW.
As in a BAW, the factors d and .rho. are essentially
during operation. However, the elastic stiffness, c, may vary as the result of other factors, thus altering the frequency of the SAW.
In both BAW and SAW devices, a mechanical mode shape is, in part, determined by the elastic stiffness, c, of the piezoelectric substrate. In turn, the mechanical mode shape when combined with all the stresses exerted on the piezoelectric substrate determines the precise frequency at which the device will operate.
Under static conditions all the stresses applied to the piezoelectric substrate are accounted for by the mechanical mode shape which remains constant during operation. However, additional stresses can be introduced when the piezoelectric substrate is accelerated, decelerated or vibrated. Since high precision circuitry using acoustic vibrators is often needed in violently moving vehicles, such as helicopters, compensation for external acceleration becomes critical. Since acoustic vibrators are also used in accelerometers, variable sensitivity is necessary to quickly adjust to rapidly changing external conditions.
The prior art teaches a number of methods of compensating for stresses caused by external acceleration. One technique is to add or delete material from the electrodes altering the mechanical mode shape. Examples of such techniques are found in U.S. Pat. Nos. 4,837,475 and 4,836,882.
Another technique uses alteration of the operating frequency to change the mechanical mode shape and thus compensate for external stresses. An example of this technique is found in the publication by Ballato et al entitled, "ELECTRONIC DESENSITIZATION OF RESONATORS TO ACCELERATIONS", Forty-Fourth Annual Symposium on Frequency Control, U.S. Army Electronics Tech. & Devices Laboratory, pp. 444-451.
In yet another technique, additional multi-electrode structures are used to vary mechanical mode shape and thus alter sensitivity to external accelerations. An example of this technique is found in the publication by Smythe & Horton entitled, "ADJUSTMENT OF RESONATOR G-SENSITIVITY BY CIRCUIT MEANS", Forty-Fourth Annual Symposium on Frequency Control, Piezo Technology, IEEE (1990), pp. 437-443.
A further technique includes calculating optimal electrode masses and configurations for certain types of acceleration, and limiting the design of the acoustic vibrators to only those configurations having the least acceleration sensitivity. An example of this technique is found in the publication by Lee and Guo entitled "ACCELERATION SENSITIVITY OF CRYSTAL RESONATORS AFFECTED BY THE MASS AND LOCATION 0F ELECTRODES", Forty-Fourth Annual Symposium on Frequency Control, Dept. of Civil Engineering & Operation Research, IEEE (1990), pp. 468-473.
Each of the aforementioned systems suffers from lack of flexibility. For some there are severe limitations in usable frequencies or electrode configurations. Rapidly and easily altering acoustic vibrator sensitivity is problematical for all of them.
A more adaptable system providing rapid compensation to external stresses is taught in U.S. Pat. No. 4,453,141, to Rosati, using a dynamic suppression system for doubly-rotated cut quartz BAW resonators. This system detects externally induced vibrations on the piezoelectric substrate and generates an electrical signal which is a replica of the vibration acting on the substrate. The electrical signal is thereafter modified and applied directly to the RF electrodes to immediately compensate for the externally induced vibrations. An isolation network is necessary for the proper operation of this system, as is a doubly-rotated cut quartz substrate. The Rosati system depends on "electroelastic" operation and so cannot be used with singly rotated cut quartz. This system is also not adaptable to SAW devices, and cannot use many electrode configurations.